1. Field of the Invention
The invention relates to a method for transmitting packet data in a radio communication system and a radio communication system, in particular a mobile radio system, for carrying out the method.
2. Description of the Related Art
In a radio communication system, messages and data, for example voice, image information or other data, are transmitted via a radio interface by electromagnetic waves. The electromagnetic waves are emitted here with carrier frequencies which lie in the frequency band which is provided for the respective system. A connection is set up via a radio interface between at least one base station and a plurality of subscriber stations which may be mobile or fixed radio devices and which are the lowest elements of the system. A base station supplies the radio network over a region which is up to several square kilometers in size and referred to as a radio cell. The spatial delimitation of a radio zone which can be achieved in this way enables the scarce carrier frequencies to be reused at a certain distance at the same time without the channels interfering with one another. For this purpose, a plurality of radio cells form a cluster which is administered jointly by a base control station. A plurality of base control stations in turn are connected via a mobile switching device via which the access to the fixed network or further mobile radio systems is carried out.
In order to provide better coverage of the demand for available spectrum space in the scarce “carrier frequency” resource, synchronous multiplex methods on the basis of a frequency-selective, time-selective or spread-code selective multiple access method for distributing the transmission capacity of a channel between a plurality of connections have been introduced. For this purpose, a predefined pattern made up of transmission-synchronous and reception-synchronous frequency bands, time slots or code sequences is used in agreement between the transmitter and receiver, including possible switching nodes. The transmitters assign the data of the individual connections to this pattern, and the receivers separate the data determined for them from the received data stream.
A further basic method is packet switching. It is based on the common use of a transmission channel with a high transmission capacity by a plurality of different connections. The transmission channel on the time axis is also divided up for this method, however not into fixed time slots but rather into addressed data packets of variable length, it being possible to transmit simultaneously even a plurality of data packets by additional separation in code or frequency. The data is transmitted variably with respect to time, for which reason the term asynchronous multiplex method can also be used. Each transmitter can access at any time the transmission capacity unused until then, and can transmit its data using, for example, the ALOHA method, a stochastic access method. In addition, the data rate of a connection can be varied in a very flexible way. The transmitter can influence the data rate both in terms of the time intervals at which it transmits the data packets and in terms of the length, and the number of codes or frequencies used.
Future mobile radio systems such as the UMTS (Universal Mobile Telecommunications System) will provide network subscribers with a multiplicity of different services with different bit rates. In addition to the pure transmission of voice, multimedia applications with the associated variety of services will make up a large part of the data volume. The transmission of packet data in a way which is variable over time on the radio interface, permitting very flexible access in the time slot pattern or code pattern which is predefined by the synchronous time-division multiplex component of UMTS, is particularly promising here. At the same time, stations wishing to transmit may start to transmit at specific synchronous points in time when transmitting in the uplink direction using what is referred to as the “slotted ALOHA” method. In the downlink direction, the distribution of resources is performed by the base station.
Large data packets cannot be transmitted in one piece. For this reason, before the actual transmission, they are once more divided up into small data units, referred to as PDUs (Protocol Data Units). Each PDU contains user data and is—generally—provided with a header which contains the associated receiver address and sender address, the serial number of the PDU and possibly further information necessary for the transportation of the PDU. In the case of packets in the field of mobile radio, the receiver address and sender address is typically not always present in each header. The serial number or sequence number is an identification number for directly identifying a specific PDU. At the end of the user data, a checksum, which is calculated by an error-detecting code (CRC=Cyclical Redundancy Check Code) relating to the user data bit, is added for the detection of errors which arise on the transmission link. Known headers and CRCs have a constant length. In the case of UMTS, RLC-PDUs have a header which can be prolonged—signaled by a marker (flag)—by a header extension. If the length of the PDUs is constant, the end of the PDU is defined at the same time as the start of the PDU. On the other hand, if the length of the PDU is variable, the length of the PDU is specifically stated in the header or the end of the PDU is identified, in precisely the same way as the start of the PDU, by an additional marking (flag) on a link layer.
The entire PDU, composed of header, user data and CRC, is additionally channel-coded before transmission in order to increase the error protection on the transmission link. More redundancy is introduced into the system by various code algorithms. An additional error protection against drops in level is also achieved by bit interleaving. Here, the PDU which has already been channel-coded, referred to as CPDU (Coded Protocol Data Unit) is either distributed in chronological succession, simultaneously or partially simultaneously or successively. The CPDUs are then transmitted via the radio link. The abovementioned components of a PDU are distributed here over all the CPDUs by error protection coding and methods in each case.
The mobile radio channel is one of the most unfavorable transmission channels occurring in telecommunications technology. One problem is short-term drops in reception level and signal distortions which are mainly caused by multipath propagation (fast fading) of the radio waves which are reflected or bent at obstacles on the transmission path. The transmission signal reaches the receiver with time offset on paths of different lengths, which, depending on the phase angle of the signals, leads to extinctions or amplifications (interference) at the receiver. In the case of transmission of the abovementioned data structures, it is possible for this to result not only in loss of data, but also with equal probability, in loss of the header. As a result, a PDU can no longer be assigned to a connection.
When data is lost on the transmission path, which loss is noticed by the receiver in the evaluation of the checksum of the CRC, according to the known prior art there is the possibility of immediately repeatedly transmitting one or all of the PDUs of a data packet by a repeat request (ARQ or Automatic Repeat Request) from the receiver. To do this, the transmitter administers a copy of the transmitted PDU or PDUs of a data packet until it has received a reception confirmation for the respective data unit. The receiver for its part requires a sufficiently large memory to be able to store error-free PDUs until all the PDUs received with an error have been repeated, and the message can be assembled.
The receiver will therefore request a PDU once more by a protocol structure. However, as it is highly probable that the header of the PDU with an error cannot be decoded, and the receiver is thus not able to repeat the correct PDU to the transmitter, there are concepts in which the header is not transmitted with the user data in a traffic channel but rather separately in a control channel, for example during transmission to the FACH (Fast Associated Control Channel) in the downlink, with a relatively high degree of error protection. As this control channel is under certain circumstances not in the same timing configuration as the traffic channel, the control channel requires additional identification signaling in order to be able to discover the subscriber in the system and be able to assign the header to him. Furthermore, this control channel is used simultaneously with a finite capacity of a large number of subscribers in the system, which can lead to overloading of the resource.
In digital radio communication systems it is particularly critical if the identification number or sequence number of a PDU is lost during the transmission. Specifically in ARQ error correction methods with a repeat request by the receiving station, sequence numbers are used to permit the receiver side to request supplementary information for correcting incorrectly transmitted PDUs. In what is referred to as the “Hybrid-ARQ I”, the receiver end informs the transmitting station, which may be in particular a transmitter, directly or indirectly of the unsuccessfully transmitted sequence numbers or the sequence numbers of the unsuccessfully decoded PDUs, which are then sent once more by the transmitting station.
In other error correction methods such as what is referred to as the “Hybrid-ARQ II” method and what is referred to as the “Hybrid-ARQ III” method, an incorrectly received PDU (first coding unit) is linked to a supplementary item of information (second, third, . . . , n-th coding unit) which is subsequently transmitted by the receiver, in order to restore the PDU. The term “coding units” refers below in this context to the data sets, in particular information sets or redundancy sets which are generated from data packets and which permit the data packets or PDUs to be restored at the receiver end either individually or by suitable linking. It is possible to send once more coding units which have already been dispatched or sent and to combine them with the already transmitted version by combination in the best possible ratio (maximum ratio combining). For example, in ARQ II/III methods, coding polynomials can be used, in which what are referred to as “rate matching” measures are not excluded. For this reason, it will be ensured in ARQ II/III methods or comparable methods that the sequence number of the receiver is also reliably recognized as being error-free or having an error when the PDU itself is faulty. Furthermore, the transmission protection of an individually transmitting sequence number will be of appropriate quality in comparison with the quality of the transmission protection of the data. Here, under certain circumstances, it will also be possible to take into account the gain in multiple transmission with increasing coding depth of the data.
In Hybrid-ARQ II/III methods or comparable methods it is therefore currently necessary for the coding units which are transmitted in with an error to be stored in the receiver in order to be linked to the following corresponding coding units. The assignment of these two coding units, or of a plurality of respectively associated coding units, can be carried out using the sequence number. As a result of the subsequent dispatching, the sequence of the dispatched sequence numbers is not fixed from the outset. In particular, as a result the sequence numbers do not necessarily increase monotonously. The sequence numbers of the incoming PDUs are therefore explicitly communicated to the receiver. For Hybrid-ARQ II/III or comparable methods it is therefore desirable for the sequence number also to arrive correctly at the receiver with a high degree of probability even if the associated PDU data is faulty. Problems may result in particular if the sequence number is dispatched together with the PDU: the correctness of transmitted data can be checked by reference to a CRC sent with the data. If the CRC also refers to the sequence number, a positive CRC check result confirms the correctness of PDU and sequence number, while a negative output of the CRC check result indicates an error in the PDU or in the associated sequence number or header with the sequence number. Given a negative CRC check result it is thus unclear whether the sequence number or the header with the sequence number has been correctly or incorrectly received or decoded. If the receiver knows the sequence number of a faulty PDU, it can be specifically requested. As the knowledge of the sequence number of faulty PDUs for Hybrid-ARQ II/III methods or comparable methods is therefore necessary to request a repeat transmission, it is necessary to select a form of the transmission of the sequence numbers in which the sequence numbers are transmitted reliably enough and in which the correctness of the sequence numbers can be determined by CRC check methods independently of the correctness of the transmitted PDUs.
In Hybrid-ARQ II/III methods or comparable methods, it is also necessary to signal successfully to the receiver whether the coding unit which is possibly transmitted in a disrupted fashion is the first, second, third, . . . or n-th coding unit. The correctness of the transmitted coding unit number (1, 2, 3, . . . or n) cannot be checked together with the data by a CRC. The correctness of the received coding unit number must also be capable of being checked when the transmission of PDU data is disrupted.
Hitherto, there have been the following approaches for solving the problems formulated above:    A) For Hybrid ARQ I, the simplest possibility is for the receiver end to send the sender or transmitter just one status message relating to the successfully received PDUs. The sender can read directly from this which of the dispatched PDUs has not been successfully received by the receiver. It is necessary to ensure here that the PDUs which have not been confirmed would have had to have already arrived at the receiver at the dispatching time of the status message, and have consequently clearly been received faultily so that renewed transmission is appropriate. A reliable reception of the sequence numbers of disrupted PDUs would significantly simplify the ARQ I method and would have the advantage that in the case of a disruption of the transmission channel the receiver can request re-dispatching of the disrupted PDUs explicitly without an unnecessary loss of time.            For ARQ I/III methods or comparable methods, a transmission of the sequence numbers which is improved with respect to the data is absolutely necessary as during the decoding it must be ensured that the coding units which are combined for this purpose are associated with the same PDU.            B) In future mobile radio systems, for example UMTS (Universal Mobile Telecommunications System) systems, it would in principle be possible, when making a UDD (Unconstrained Delay Data bearer service) downlink transmission, i.e. when connecting in the downlink direction from a hierarchically superordinate station to a subordinate station, to use in the reply mode (“acknowledged mode”), the FACH for reliable transmission of the sequence number and, if appropriate, of the coding unit number (for example coding unit number: first, second or n-th transmission). Data in the FACH are as a rule better protected than the data in the traffic data channel (TCH) as a result of better coding, and in the case of the TDD (Time Division Duplex) method by interleaving over two frames and by virtue of maximum transmission power. The transmission of the sequence numbers using the FACH has, however, the disadvantage that large FACH capacities have to be made available for it. Unused FACH capacities are resources which are wasted in systems with what are referred to as hard blocking. Owing to the high transmission power of FACHs, unnecessarily high interference is additionally generated.
The transmission of the sequence number in the traffic channel may provide the possibility of each user individually regulating the power for the transmission path. In addition, in contrast to an omnidirectional FACH, the use of direction-dependent emissions in the direction of the receiver unit can improve the capacity utilization of the system.